As a method for carrying out combustion while holding the emission of pollutants to a low level, fluidized bed combustion is advantageous over other methods of combustion in many points. It nevertheless suffers from certain disadvantages.
First, since this method uses limestone as the fluidizing solid, the SO.sub.x produced in consequence of combustion of fuel containing sulfur, though removable to some extent, is removed with poor efficiency as compared with the method of wet flue gas desulfurization.
When fuel containing ash is burned within the fluidized bed, part of the ash is converted into a slagging material because of the intense heat and is deposited on the bottom of the fluidized bed. This heavy ash must be removed from the bottom of the fluidized bed but during removal it is impossible to avoid discharge of the hot fluidizing solid from the fluidized bed. Thus, the removal not only impairs the thermal efficiency of the operation but also proves disadvantageous from an economic point of view.
The greater part of the ash produced on the fluidized bed is entrained by the waste gas and discharged from the furnace interior. Since this ash contains fine particles which cannot be thoroughly arrested by the cyclone ash collector alone there is no way to obtain efficient removal of this ash in the dry method other than to use a bag filter or an electric precipitator. The ordinary bag filter suffers from heavy pressure loss and generally has a low working temperature not exceeding 200.degree. while the heat-resistant bag filter is expensive. Moreover, electric precipitators are very expensive, though they have low pressure loss and provide outstandingly efficient recovery of dust.